Polarized infrared spectra of CH4 adsorbed on NaCl(100) in the temperature range 4–47 K are reported. Coverage-dependent data indicate that there is only one kind of adsorption site. The adsorbate layer is also seen to grow in the form of constant density islands. A partial degeneracy breaking into doublets is observed in both the ν3 and ν4 infrared-active vibrations. Analysis of the ν4 doublet reveals one transition dipole component perpendicular to the (100) face of NaCl and the others in the plane of the face. Isotopic dilution experiments of CH4 in CD4 show that some of the ν4 doublet separation can be attributed to resonant or dynamic adsorbate–adsorbate interactions. The remaining splitting cannot be specifically assigned to any particular interaction but must involve static adsorbate–adsorbate coupling and surface–adsorbate interactions. A structural analysis consistent with the spectroscopic data and the symmetry of slab groups is developed. Two possible adsorbate geometries are considered. A unique determination of the adsorbate geometry will have to await other surface measurement techniques.

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We have measured polarized infrared transmission spectra of a variety of molecules physisorbed to NaCl(100). Here we discuss the relation between vibrational spectroscopy of an adsorbate and the geometrical structure of a monomolecular layer. Spectra and proposed structures of CO, COz, C2H2 and CH,, layers on NaCl(100) are presented as examples.

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